Fix an API loophole introduced with commit 9791554b45a2 ("MIPS,prctl:add PR_[GS]ET_FP_MODE prctl options for MIPS"), where the caller ofprctl(2) is incorrectly allowed to make a change to CP0.Status.FR orCP0.Config5.FRE register bits even if CONFIG_MIPS_O32_FP64_SUPPORT hasnot been enabled, despite that an executable requesting the moderequested via ELF file annotation would not be allowed to run in thefirst place, or for n64 and n64 ABI tasks which do not have non-defaultmodes defined at all. Add suitable checks to `mips_set_process_fp_mode'and bail out if an invalid mode change has been requested for the ABI ineffect, even if the FPU hardware or emulation would otherwise allow it.

Always succeed however without taking any further action if the moderequested is the same as one already in effect, regardless of whetherany mode change, should it be requested, would actually be allowed forthe task concerned.